Abstract/ Project Summary Many organic molecules aggregate at micromolar and sub-micromolar concentrations; these colloidal aggregates are the dominant artifact in early drug discovery and chemical biology. The colloids form a state intermediate between true solution and precipitate, with properties that differ from either. They inhibit both soluble and membrane proteins by direct binding followed by partial denaturation. These are the conclusions of the first decade of this program of research. There is no modern screening group that is unaware of colloidal aggregation, and all use the techniques developed by this project to detect them. A surprise of the last period was the range of conditions in which the colloids were stable, from cell culture, to plasma, to simulated intestinal fluid. In the next period, we investigate colloidal aggregates in cellular assays, drug repurposing, drug formulation, and animal pharmacokinetics. More ambitiously still, we attempt to optimize these unusual particles for stability and antibody loading, for delivery. The specific aims are: 1. The impact and mechanism of colloidal aggregation at increasing levels of biological complexity. We investigate a. the persistence of colloid aggregation among late stage clinical candidates, collaborating with Novartis. b. Similarly, we test for aggregation of about 100 likely-to-aggregate drugs in repurposing libraries. Experience with itraconazole suggests that repurposing screens can mislead all the way to clinical trials, if uncontrolled for mechanism. c. We continue to investigate the structure and mechanism of aggregates, using SAXS to explore whether they are hollow or filled, whether they adsorb or absorb proteins, whether they are micro-crystaline or fluid, and their characteristic inhibitory incubation effects. Finally, we investigate d. The effect of aggregation on whole animal pharmacokinetics. 2. Exploiting of colloidal aggregates for protein delivery. An unexpected observation from the last period was that colloidal aggregates could be co-formulated for stability, homogeneity, and protein loading. Exploiting this observation, we investigate a. Colloidal drug antibody conjugates that specifically deliver drug payloads to cells. In these particles, the delivery vehicle?the colloid?is almost 100% drug. Cell delivery deposits 107 drug molecules per colloid. b. optimization of co-formulated colloids for stability, protein loading, size, and mono-dispersity, each key to pragmatic use of the particles. Finally, we c. Investigate these colloid antibody conjugates as vehicles to deliver large drug payloads to tumors in a mouse model. Extensive preliminary results support the feasibility of these studies.